Technology for positioning mobile radio terminals using the signals received from one or more transmitters has been widely used for many years. Such systems include terrestrial networks of transmitters (e.g. Loran) and networks of satellites (e.g. GPS and Galileo) deployed specifically for the purpose of locating the receiver, as well as methods that use general-purpose radio networks such as cellular mobile telephone networks (e.g. WO-A-97-11384) or TV and radio transmitter networks. (e.g. EP-A-0303371).
Within a cellular mobile telephone network, for example, the position of the terminal may be determined based on the identity of the serving cell, augmented by information such as the round trip time delay between the serving transmitter and terminal, the strengths of signals received from the serving and neighbouring transmitters, or angles of incidence of received signals. In such cases, the calculated position usually has a large error associated with it which can range from several hundreds of meters to many kilometers, depending on the density of the cellular transmitter network. An improved position may be obtained using the observed time difference of arrival (OTDA) of signals received at the terminal from two or more network transmitters.
OTDA methods give good position accuracy using only the signals available within the cellular radio network. However, they require the precise transmission time offsets between transmitters to be determined in order to solve the positioning equations. This has been done using location measuring units (LMUs) having additional receivers. LMUs are placed at known locations so that their OTDA measurements can be converted directly into a network timing model (see for example WO-A-00-73813).
Alternatively a technique and system (see WO-A-00-73814 and WO-A-00-73813) may be used in which measurements of signals from a number of geographically disparate cellular transmitters at known positions made, for example, by two or more geographically disparate terminals at unknown positions, may be used to compute both the positions of the terminals and all the timing offsets between the measured transmitters, without the need for LMUs. Terminals in the network make measurements of the relative time offsets of signals they receive from network transmitters and they send these to the computing node of the system (sometimes known as the ‘Serving Mobile Location Centre’, SMLC). The system maintains a timing model defining the timing relationships between transmitters in the cellular radio network using this background stream of terminal measurements, and continuously adjusts itself for the slow timing changes in the network. The system also computes the positions of the terminals.
The problem addressed by the present invention is that of determining whether a terminal is within a given locale, say within a few hundred meters of a given position, or whether it has moved outside of it. This could obviously be done by using any of the methods mentioned above to determine the position of the terminal. The calculated position is then compared with the given position. However, such a calculation (for example carried out in an SMLC remote from the terminal) generally has an appreciable communications overhead associated with it, both in the sizes of messages sent and received and in the latency times. In particular, it could take several seconds, or even tens of seconds, for the user of a terminal to establish whether or not he or she was within the given locale, which could hinder the commercial exploitation of a home-zone system. We have appreciated that for such uses it is unnecessary for a terminal's position to be calculated, merely the change in its position, and this can be achieved without the hindrance of the communications overhead.
U.S. Pat. No. 5,969,679 describes a method and apparatus for determining whether a wireless station is operating within a prescribed geographical region, specifically a terminal in a CDMA wireless communication system. One of the characteristics of a CDMA system or network is that the transmission times of the network's transmitters are predefined, unique to each transmitter and fixed, i.e. synchronised, and a terminal in such a system can determine the transmission times of the signals it receives from the transmitters by decoding the CDMA signals from the base station.
Unfortunately, in GSM or WCDMA communication systems, which comprise the majority of the cellular communication networks in use in the world today, the network's transmitters are unsynchronised with each other and with respect to any common timing reference, and the transmission times can also drift slowly over time. This means that in, for example GSM or WCDMA communication systems, the technique used in U.S. Pat. No. 5,969,679 (involving the use of the propagation delays and the difference in propagation delay between two transmitters) is unavailable because the propagation delays and the difference in propagation delay cannot be determined from the base station transmissions in such communication systems.
The problem to be overcome therefore is that of identifying one or more movement indicators which reflect movement of the terminal within the network without requiring that the network signal transmitters be synchronised or that the propagation delays be calculated.